1. Field of the Invention
The invention relates to a stabilizing method for a current source, and more particularly to a stabilizing method for a current source which provides a current varying with temperature.
2. Description of the Related Art
For integrated circuit design, reference voltages and reference currents are required, wherein the reference voltages and the reference currents are usually included in a bias part of the integrated circuit. For general applications, the bias part of an integrated circuit is designed according to operating temperature of the integrated circuit. However, variations in operating temperature are not considered for the design of the bias part.
During the operation of integrated circuits, operating temperature varies according to ambient temperature variation or heat generated by electronic elements within the integrated circuit. Operating temperature variations may affect signal transmitting operations of the integrated circuit so that the transformed signals have noise due to the operation temperature variation. For example, an analog-to-digital converter is affected by temperature noise. Moreover, a microprocessor with a sensor is more sensitive to temperature variations, thus, temperature variations also affects operations of microprocessors with sensors.
In general, bipolar junction transistors (BJTs) are used to design integrated circuits having temperature variation. There is a logarithmic relationship between base-emitter voltage VBE and collector current IC of a BJT and the base-emitter voltage VBE is affected by temperature variation. The relationship between the base-emitter voltage VBE and the temperature variation is represented by the following:VBE(H,IC)=EGE−H(EGE−VBEN)+VTHH log(IC/IN)−ηVTHH log H  (Function 1)
wherein, H=T/TN, and T represents absolute temperature, and TN represents standardized temperature. TN is usually a middle value of an operating temperature range, such as 300K (27°). EEG represents an assumed value of the base-emitter voltage VBE at absolute zero (zero degree Kelvin), or about 1.14V to 1.19 V. VBEN represents a value of the base-emitter voltage VBE when junction temperature of a BJT is equal to the specific value TN and collector current IC is equal to a specific value IN. VTN represents a value of thermal voltage(=kT/q) at the standardized temperature TN. η represents a curve constant, about 2 to 4.
FIG. 1 shows a line diagram of Function 1. Referring to FIG. 1, showing characteristics of BJTs, the base-emitter voltage VBE decreases when temperature rises and increases when collector current IC increases. BJTs are usually applied in circuits, wherein when there is a rise in temperature, current increases, achieving current balance so that the current remains at a constant value.
However, since diodes are required in a BJT circuit, requirement for a BJT circuit increases hardware costs and device/element volume. Thus, it is desired to provide an alternative method for stabilizing a current source.